1. Field of the Invention
The present invention relates to the technology field of electronic circuits, and more particularly to a control circuit for reducing power loss of LLC resonant converter during light-load or no-load operation.
2. Description of the Prior Art
Technologies of switching-mode power supply (SMPS) have been widely applied in manufacture of power supplies for various electrical apparatuses and electronic products. Moreover, since there has a tendency toward developing small-size and light-weight electronic products today, power density of the SMPS needs to be enhanced through raising switching frequency in order to effectively shrink the SMPS's framework volume. However, in practice, even though to raise switching frequency can indeed facilitate the SMPS use small-size magnetic components and capacitors, this way would simultaneously cause the incensement of switching loss produced by power switches in the SMPS, and also bring the SMPS into being subject to electromagnetic interference (EMI) more easily.
Accordingly, LLC resonant converter with advantages of zero voltage switching (ZVS) and zero current switching (ZCS) is hence developed and proposed. FIG. 1 shows a circuit framework diagram of a conventional LLC resonant converter. As FIG. 1 shows, the conventional LLC resonant converter 2′ comprises: a power switch unit 23′ coupled with a DC source VDC′, a resonator unit 24′, a transformer unit 25′, an output rectification unit 26′, and a low-pass filter unit 27′, wherein a closed loop controller module 1′ is electrically connected between output terminals of the LLC resonant converter 2′ and the power switch unit 23′. Moreover, FIG. 1 also indicates that the said closed loop controller module 1′ mainly comprises a signal detection unit 11′, a controller 12′, and a driving unit 13′.
For making the LLC resonant converter 2′ steadily supply an output current/voltage to a load 3′, a first switch controlling signal and a second switch controlling signal are generated by the closed loop controller module 1′ and alternately inputted to a first power switch and a second power switch in the power switch unit 23′. Herein, it needs to further explain that, there is a spacing time called “dead time” existing between the first switch controlling signal and the second switch controlling signal. Moreover, when the LLC resonant converter 2′ works under a light-load operation, the closed loop controller module 1′ is configured to immediately stabilize the output current/voltage of the LLC resonant converter 2′ by raising the switching frequency of the power switch unit 23′. However, the output voltage of the LLC resonant converter 2′ would go up with occurrence of parasitic stray capacitance during the high frequency switching of the power switch unit 23′. Therefore, for the purpose of reducing switching loss of the LLC resonant converter 2′ during light-load or no-load operation, researchers and power supply manufacturers are developed and proposed several improvement approaches.
First of all, at least one dummy load is connected to the output terminals of the LLC resonant converter 2′ for attenuating the parasitic stray capacitance during light-load or no-load operation. However, it is a pity that the addition of the dummy load does not only enlarge the whole framework volume of the LLC resonant converter 2′, but also lead to reduction of the power conversion efficiency of the LLC resonant converter 2′. On the other hand, second improvement approach utilizes a burst mode controller connected to a primary side of the transformer unit 25′ to control the LLC resonant converter 2′ to work at an burst mode during light-load or no-load operation. In burst mode, the switch controlling signal are particularly set to comprises at least one nearly-fixed frequency switching period consisting of a series of switching cycles and at least one long idle period, wherein the power switches are in OFF-state or the duty cycle of the switch control signal is set to zero during the long idle period. By implementing the second improvement approach into the LLC resonant converter 2′, the average switching frequency can be considerable lowered, thereby reducing the switching loss of the LLC resonant converter 2′.
However, the above-mentioned second improvement approach still exhibits a primary drawback, that is, the LLC resonant converter 2′ would produce a noise with audio frequency when the power switch unit 23′ works at the burst mode based the controlling of the burst mode controller. Furthermore, when applying a third improvement approach to the LLC resonant converter 2′, the closed loop controller module 1′ is configured to treat the first switch controlling signal and the second switch controlling signal inputted to the power switch unit 23′ with a variable-frequency (VF) process and/or a phase shifting (PS) process. It is easy to presume and understand that, multi types of circuit chips must be added into the closed loop controller module 1′ in order to implement the third improvement approach into the LLC resonant converter 2′, such that both complexity of the closed loop controller module 1′ and whole manufacturing cost of the LLC resonant converter 2′ raises.
From above descriptions, it is clear that there is no an ideal improvement approach or solution for effectively solving the phenomenon of switching loss occurring as an LLC resonant converter is operated under light load or no load; in view of that, inventors of the present application have made great efforts to make inventive research thereon and eventually provided a control circuit for reducing power loss of LLC resonant converter during light-load or no-load operation.